1. Field of the Invention
Exemplary aspects of the present invention generally relate to a light source unit, an optical scanner including the light source unit, and an image forming apparatus including the optical scanner.
2. Description of the Background Art
Conventionally, known image forming apparatuses such as a digital copier, a facsimile machine, and a printer include an optical scanner that illuminates a latent image bearing member, such as a photoreceptor, with light to form a latent image on the surface of the latent image bearing member. The light projected from the optical scanner scans the latent image bearing member in accordance with image data. Then, the latent image is developed with toner to form an image.
Generally, such an optical scanner includes optical elements such as a coupling lens, an aperture, a cylindrical lens, a polygon scanner, a focusing lens, and so forth. The coupling lens forms a light flux projected from the light source into parallel rays of light. The aperture shapes the parallel rays of light from the coupling lens into a desired shape. The cylindrical lens transforms the light into a line image. The polygon scanner serves as a deflector that deflects the light. The focusing lens focuses the light deflected by the polygon scanner on the surface of the latent image bearing member.
The optical scanner also includes a synchronous detection device that determines a start position for light to start writing. The optical elements and the synchronous detection device are enclosed in a single optical housing.
The coupling lens not only forms the light flux from the light source into parallel rays of light, but also adjusts an optical axis of the light flux. In particular, the coupling lens adjusts any divergence of the light flux from the light source, any positioning error of the light source, and any variation in the point of light projection in order to obtain an optimum parallel light flux and an optimum position for the optical axis. For this reason, in general, the position of the coupling lens cannot be fixed in advance, and thus the coupling lens is not positioned by a positioning member. Consequently, the coupling lens is fixed to a holder using an adhesive agent after the above-described adjustment is performed.
Although advantageous, since the coupling lens is not fixed by the positioning member but is adhered to the holder using the adhesive agent, the position of the coupling lens fluctuates due to degradation of the adhesive agent over time and expansion/contraction of the adhesive agent caused by environmental changes including, but not limited to, fluctuations in temperature.
Such fluctuation of the position of the coupling lens causes the position of the optical axis to shift undesirably. Further, divergence of transmitted light fluctuates, thus resulting in fluctuation of a focal point, which then causes fluctuation of the position of the light beam on a scanned surface, color drift, and degradation of dot reproduction. Ultimately, the latent image cannot be formed properly.
In order to address such a difficulty, in one related-art approach, an adhesive agent is applied to a certain area (an adhesive agent application area) of the coupling lens. The adhesive agent application area is formed on a portion of the peripheral surface of the coupling lens. The projected length of an adhesive layer formed on the adhesive agent application area is longer in a sub-scanning direction (a horizontal direction) than a main scanning direction (a vertical direction). With this configuration, an amount of shift of the coupling lens in the sub-scanning direction is less than in the main scanning direction, thereby reducing an amount of shift in the pitch of the scan line on the scanned surface.
Although advantageous, there is a drawback to such a configuration in that, because the position of the coupling lens is fixed by the adhesive agent without a positioning member, the position of the coupling lens is not securely fixed. In other words, the coupling lens can still move due to degradation of the adhesive agent over time and as well as due to expansion/contraction of the adhesive agent caused by environmental changes, for example, fluctuations in temperature. Therefore, a more secure positioning method is required.